This invention relates to a method of incorporating radioactive iodine into an inert solid material for long-term storage.
One of the concerns of the nuclear energy program is the recovery and storage of radioactive iodine from various liquid and gaseous process streams to prevent its entry into the environment and potential damage to human and animal life. Chemical species which contain radioactive iodine include inorganic species such as elemental iodine and organic species such as methyl iodide. These may be present in reactor containment vesels from normal reactor operation and in particular in the case of fuel element cladding failure. They are also present in waste gas streams from fuel element dissolver operations in nuclear fuel reprocessing plants and in the aqueous waste streams resulting from the fuel reprocessing. Radioiodine is formed in the nuclear reactor fuel by the fission of the fuel material. Although there are other radioactive isotopes of iodine, only .sup.129 I (t.sub.1/2 =1.7.times.10.sup.7 yrs) has a very long half-life. Therefore long-term storage must be provided for this isotope.
A number of processes have been developed for the recovery of radioactive iodine from the off-gas streams of a fuel reprocessing plant. Caustic scrubbing has been used at several reprocessing plants, but has been found ineffective for the removal of organic iodides. A mercuric nitrate - nitric acid scrub system (Mercurex process) which employs a recirculating solution through a packed tower has also been used. However, the system produces substantial volumes of waste and no suitable methods have been developed for removing the iodide as a solid, reducing waste volume and recycling the mercury. The removal of iodine species from a gas stream that employs nitric acid as the liquid scrubbing medium in a bubble-cap tower is also under development (Iodex process). The process effectively removes all iodine species and oxidizes them to a nonvolatile iodate form. This process is advantageous in that the iodine wastes as HI.sub.3 O.sub.8 can be concentrated and disposed of directly in metal canisters or incorporated into a cement matrix in the form of barium iodate. Off-gas containing the iodine species can also be passed through a silver-exchanged zeolite where the iodine reacts to form chemisorbed silver iodate. Because of the high cost of silver and its value as a resource, the iodine should be removed and the silver-exchanged zeolite regenerated by using hydrogen to carry the iodine to less expensive lead-exchanged zeolite for long-term storage.
However, none of the processes completely solve the problem of immobilizing the radioactive iodine for long-term storage. For example, containers of radioactive material ultimately corrode, releasing any soluble or volatile material to the environment. Cement is subject to leaching and will eventually release the radioactive material. Thus none of the state of the art processes have been demonstrated to be suitable for terminal storage or disposal of radioactive iodine because of their instability, solubility or dispersivity. Conversion of the radioiodine to a monolithic insoluble form of low surface area suitable for emplacement in either geological storage or other long-term disposal facility is necessary.